The present invention relates to a metal cathode sheet, more particularly a metal cathode sheet having an edge protector.
In the refinement of crude metals with the aid of electrolysis for recovering pure metal, the metal is dissolved in an electrolysis tank from the impure anode and deposited in pure form on the cathode. The impurities remain dissolved in the electrolyte or form anode slime.
Various constructions of electrolysis cathodes are counted among the related art. They differ mainly in the choice of material or material combination of bearing rail and metal cathode sheet with a view to relatively good corrosion resistance, mechanical stability, and electrical conductivity for minimizing energy losses.
Typically, the side edges of the cathodes, which are vertically aligned in the electrolysis tank, are provided with an electrically insulating screen as edge protection. This helps to prevent the growing together of the metal layers deposited on both sides of the metal cathode sheet from reaching over the side edges.
In this connection, it is known to coat the side edges with wax. The disadvantage of doing this is, first of all, that a large quantity of wax is required. Furthermore, if the wax is interspersed with contaminating particles, bridge formations with the electrolyte can take place anyway, and this may lead to uncontrolled growth of metal buds whereby the depositing performance drops and the course of the operation is disturbed. Metal cathode sheets are therefore maintained in rotation, and the metal buds are removed. This requires an operating interruption each time.
It is also known to provide the side edges of the metal cathode sheets with edge protection made of plastic.
In the metal cathode sheet known from U.S. Pat. No. 5,314,600, the edge protectors are plastic rails that surround the vertical side edges of the cathode sheet in clamping fashion. On the side edges of the cathode sheet bore holes are provided into which holding pins are fitted, thereby fixing the plastic rails.
The edge protector has a very slack, loose connection to the cathode sheet. This has the disadvantage that electrolyte can penetrate into the edge protector. Then, at the boring edges in the cathode sheet and at the inside sheet cutting edges, high local field densities can appear with the result that, particularly at these locations, uncontrolled metal growth takes place. After longer-term application of the cathode sheet in the electrolyte, the plastic protector can be pried apart and damaged. This causes expensive repair work or possibly a complete renewal of the edge protector.
U.S. Pat. No. 5,919,343 also describes a plastic edge strip for use as an edge protector. The strip is connected to the cathode sheet using plastic pins and fusion welding technique. Nevertheless, non-fused, faulty connection regions can result due to non-observance of construction prerequisites with respect to the parts to be connected, by non-observance of certain welding parameters, as well as by errors in preassembly. These oversites allow the passage of electrolyte and lead to uncontrolled formation of buds at the outer edge. The problem of local flux line concentration at sharp-edged borings in the cathode sheet, with its negative effects, has also not been solved.
U.S. Pat. No. 6,017,429 describes a metal cathode sheet having an electrically insulating edge protector made of plastic resistant to electrolyte. The edge profile is chemically connected to the metal cathode sheet, preferably using an adhesive or a vulcanizing technique.
Also in this particular embodiment, an intimate combination of cathode sheet and edge profile is not absolutely ensured. Thus, penetration by electrolyte under the wall of the edge profile can take place.
The object of the present invention is to create an improved metal cathode sheet for use under operational conditions. Such a cathode sheet is designed to prevent uncontrolled metal growth at the side edges. Thus, on the one hand, operational interruptions and maintenance work can be reduced and, on the other hand, deposit performance can be increased.
According to the present invention, the object is attained by providing a metal cathode sheet as a component of cathode equipment for an electrolysis tank for the electrolytic recovery of pure metals, especially copper, which is provided, at least at its side edges which come into contact with the electrolyte and are vertically aligned in the electrolysis tank, with an edge protector, wherein the edge protector is made of a ceramic material.
Preferably the edge protector is made of an oxide-ceramic material such as aluminum, zirconium or magnesium oxide. The material has great hardness, strength and insulating capacity. In addition, especially aluminum oxide has very good chemical stability.
The present invention is described in detail below, using an exemplary embodiment represented in the drawings.